This invention relates to shock tubes, and more particularly to an annular arc accelerator shock tube.
There is a continuing need for a shock tube that will simulate entry of a spacecraft or probe into the atmosphere of a planet, such as Jupiter, in order to study the effects of entry shock. Past efforts to develop shock tubes have fallen short of desired goals. An electric arc driven shock tube described by W. A. Menard, "A Higher Performance Electric-Arc Driven Shock Tube", AIAA Journal, Vol. 9, 1971, pp. 2096-2098, employs a conical configuration for the interior of the driver and a disintegratable lightweight diaphragm. The diaphragm is made of mylar in order to reduce the diaphragm opening losses and to insure a fast opening time. The driver is charged in its conical chamber to just 4 percent less than the static rupture pressure of the diaphragm. When the arc is struck, the diaphragm disintegrates. Disintegration immediately provides a large opening to reduce loss of energy to the conical wall as well as dissociation and ionization of the gas, although there is loss in energy required to break and open the diaphragm. This reduction in the loss of energy permitted an increase in obtainable shock speed with 1.0 torr initial pressure from 15 to 26 km/sec, and allowed simulation of some variables of Jupiter entry, such as temperature, but a shock speed of 40 km/sec from an electric-arc driven shock tube in 1.0 torr of inert gas continued to be unattainable until the present invention.
In a further development described by the present invention in "Development of An Annular Arc Accelerator Shock Tube Driver", Proceedings Ninth International Shock Tube Symposium, Stanford University Press, 1973, pp. 678-689, a gas from a high pressure driver flows past an annular space between a centered cathode and a section of the tube wall copper plated to serve as an anode. As the wave front of the flowing gas passes through that annular space, energy stored in a bank of capacitors is discharged through the space. The resulting arc produces a heated plasma which immediately expands and then cools, driving a shock wave down the tube. The anode section is sufficiently downstream from the cathode to permit a thin insulating diaphragm to be placed between the anode and cathode, thus preventing arc discharge until the pressure wave front has ruptured the insulating diaphragm. While such a passive discharge switch proved to be effective, the slow breaking of the insulating diaphragm disrupted the gas flow and prevented achievement of full shock velocity potential.